Noble metal electrocatalysts containing platinum are widely used in fuel cell applications. Binary catalysts, e.g., of ruthenium and platinum, have been reported to have synergistic effects in some reactions. For example, a specific activity a factor of 10 higher than for pure platinum has been reported for a platinum-ruthenium catalyst. Watanabe et al., “Preparation of Highly Dispersed Pt+Ru Alloy Clusters and the Activity for the Electrooxidation of Methanol”, J. Electroanal. Chem., 229, 395-406 (1987). Watanabe et al., disclose a method for producing a platinum/ruthenium catalyst. The method produces clusters in a colloidal dispersion, with a catalyst concentration of approximately 0.7 grams of colloid metal (platinum, or platinum and ruthenium) per liter of solution. Other processes for the production of noble metal catalysts include the use of acid treated carbon supports. Such catalysts are useful in electrochemical cells. Electrochemical cells generally include an anode electrode and a cathode electrode separated by an electrolyte. A well-known use of electrochemical cells is in a stack for a fuel cell (a cell that converts fuel and oxidants to electrical energy) that uses a proton exchange membrane (hereafter “PEM”) as the electrolyte. In such a cell, a reactant or reducing fluid such as hydrogen is supplied to the anode electrode and an oxidant such as oxygen or air is supplied to the cathode electrode. The hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
Most efficient fuel cells use pure hydrogen as the fuel and oxygen as the oxidant. The use of pure hydrogen has known disadvantages, including relatively high cost and storage considerations. Consequently, attempts have been made to operate fuel cells using other than pure hydrogen as the fuel.
In an organic/air fuel cell, an organic fuel such as methanol, formaldehyde, or formic acid is oxidized to carbon dioxide at an anode, while air or oxygen is reduced to water at a cathode. Fuel cells employing organic fuels are extremely attractive for both stationary and portable applications, in part, because of the high specific energy of the organic fuels, e.g., the specific energy of methanol is 6232-watt hours per kilogram (Wh/kg). One such fuel cell is a “direct oxidation” fuel cell in which the organic fuel is directly fed into the anode, where the fuel is oxidized. Thus, the need for a reformer to convert the organic fuel into a hydrogen rich fuel gas is avoided resulting in considerable weight and volume savings for the fuel cell system. A direct methanol fuel cell is one such fuel cell system.
Materials customarily used as anode electrocatalysts are pure metals or simple alloys (e.g., Pt, Pt/Ru, Pt˜i) supported on high surface area carbon. For example, the state-of-the-art anode catalysts for hydrocarbon (e.g., direct methanol) fuel cells are based on platinum (Pt)-ruthenium (Ru) alloys. Heretofore, the best known catalyst was Pt50/Ru50 (numbers in subscript indicate atomic ratios). Gasteiger et al., J Phys. Chem., 98:617, 1994; Watanabe et al., J. Electroanal. Chem., 229-395, 1987.
Some known processes for making noble metal catalysts produce noble metal catalyst particles that are highly agglomerated on the support, especially for carbon-supported catalysts having a high metal to carbon ratio.
A need remains for improved catalysts. It is desirable to maximize the concentration of catalyst and, for supported catalysts, to minimize agglomeration of the metal particles on the support in order to maximize the metal surface area and improve the reactivity of the catalyst produced.